Free energy and kinetics of dissolution of Sokoto rock phosphate and the implications for replenishing phosphorus in the savanna soil of Nigeria

Abstract

SummaryThe direct application of Sokoto phosphate rock to restore phosphorus in the savanna soil of Nigeria has not been very successful. The dissolution of Sokoto phosphate rock was investigated in three electrolyte solutions – 0.01 m CaCl2, NaCl and KCl – at pH range 3.5–7.0 under laboratory conditions to provide solubility and kinetic data that are required to develop guidelines for direct application in the field. The phosphate rock dissolved in the salt solutions in the order KCl > NaCl > CaCl2. Particle size and ionic strength had no significant effect on the dissolution. The standard free energy of reaction ΔG°R in an acidic solution with no basic cations was −38 kJ mol−1. If Ca2+ ions were in the acidic solution, then ΔG°R increased to 210 kJ mol−1, 170 kJ mol−1 for Na+ ions, and 107 kJ mol−1 for K+ ions in the solution. The theoretical solubility constant (Ks) calculated from the relation ΔG° = −RT ln Ks gave Ks = 106.7 in an acidic solution without basic cations, but decreased to 10−36.8 with Ca2+ ions in solution, 10−29.8 with Na+ ions, and 10−18.8 with K+ ions in solution. At pH ≥ 5.5, the dissolution was more constrained by Ca2+ ions or basic cations in solution than by availability of protons. The kinetics of the dissolution reaction was best described by a power function: Ct = atb, where Ct is the amount of P released from the rock phosphate at time t, and a and b are fitting parameters. An Elovich and a parabolic diffusion expression equally gave satisfactory fits to the dissolution data, suggesting that the rate of dissolution was limited by a combination of film‐ and intra‐particle diffusion. To utilize this rock phosphate as an effective source of P, management practices that increase Ca sinks and the supply of protons to the soil are necessary. In the savanna, increasing the soil's organic matter greatly enhances cation exchange capacity and availability of protons. The practice should provide adequate sinks for Ca2+ and the acidic environment required for the release of P from rock phosphate.